Printed circuit boards contain through holes which have been drilled and typically plated. Due to drilling and plating limitations, these plated through holes are quite large with respect to the wiring. Through holes are often filled for a variety of reasons; for example, to recapture otherwise lost real-estate, or to prevent liquids used in fabrication steps from entering the through holes.
One method of filling holes involves forcing a fill composition, typically a copper filled epoxy paste, through a mask into the holes of the printed circuit board. However, such a fill composition often bleeds between the mask and the board. Also when the mask is lifted off, paste adheres to the mask and is pulled out of the through holes. Moreover, since a typical mask can only be used once, the mask must be drilled each time a set of holes is to be filled.
Conventional hole fill materials suffer from the further disadvantage in that they are not suitable when subsequent processing steps involve electroless plating of metal, because such fill materials are typically not chemically compatible with the electroless gold plating bath. The gold electroless plating bath tends to degrade the fill material, and/or leaches the fill material into the bath which interferes with the electroless plating.
It would be desirable to have an efficient technique for reliably and completely filling holes that does not cause bleeding, and does not degrade in electroless plating baths.
The present invention provides a novel method of filling apertures in substrates, such as through holes. The method utilizes a photoimageable film, and comprises the following steps: applying a photoimagable, hole fill film over the apertures; reflowing the hole fill film to flow into the apertures; exposing the hole fill film to actinic radiation, preferably ultraviolet light, through a phototool, which preferably has openings slightly larger than the diameter of the apertures; then at least partially curing the hole fill film; and then developing the hole fill film to remove the unexposed hole fill film. An advantage of the present method is that the apertures are optionally selectively filled. After the holes are filled, the hole fill film is cured, preferably by baking.
Thereafter, the substrate is preferably subjected to further processing steps; for example, nubs of cured hole fill film are preferably removed using conventional techniques. If desired, the substrate is circuitized using conventional techniques. If desired, features are gold plated using conventional techniques.
The hole fill film preferably has solids which comprise 0 to about 20% of a thixotrope and about 80% to about 100% parts of an epoxy resin system; the epoxy resin system comprises: from about 10% to about 80% of phenoxy polyol resin which is the condensation product of epichlorohydrin and bisphenol A, having a molecular weight of from about 40,000 to about 130,000; from about 20% to about 90% of an epoxidized multifunctional bisphenol A formaldehyde novolac resin having a molecular weight of from about 4,000 to about 10,000; from about 35% to about 50% of a diglycidyl ether of bisphenol A, preferably halogenated, and having a molecular weight of from about 600 to about 2,500; and from about 0.1 to about 15 parts by weight of the total resin weight, a cationic photoinitiator. The invention also relates to circuitized structures produced according to the method.